Fullerenic materials may be synthesized using a laser to ablate graphite, burning graphite in a furnace or by producing an arc across two graphite electrodes in an inert atmosphere. Combustion of a fullerenic-forming fuel under well-controlled conditions has evolved to be an attractive method particularly for high volume production. In each method, condensable matter comprising a mixture of soot, other insoluble condensed matter, C60, C70, and higher as well as lower numbered fullerenes, and polycyclic aromatic hydrocarbons (PAH) in varying amounts is collected as a condensed solid, with the total fullerene fraction typically between 5 and 15% of the total material collected, and soot being 80%-95% of the remaining total material. Carbon nanotubes, also part of the class of fullerenic materials, can be synthesized in significant yields with the use of metal catalysts in electric arc, combustion, laser ablation or chemical vapor deposition systems. The relative abundance of multi-walled (MWCNT) or single-walled carbon nanotubes (SWCNT) depends strongly on the catalyst added. For instance, the addition of iron pentacarbonyl as a catalyst precursor to premixed hydrocarbon/oxygen allows for the selective formation of SWCNT. Between 25% to greater than 40% by weight of SWCNT in condensed material can be typically obtained, with the remainder the material being mainly iron and iron oxide.
Forming dispersions of fullerenic materials from condensed solids gathered by these synthetic routes can be difficult. Although techniques such as exfoliation, dispersion and debundling of nanotubes in solution have been reported, these techniques require selecting a specific surfactant and solvent to enhance the dispersion, in addition to applying some method of physical agitation, such as ultrasonification or centrifugation. Dispersions formed by this process, however, tend to readily agglomerate and in many cases, do not sufficiently disperse. Moreover, significant quantities of surfactant is generally required for the dispersion, which is not always compatible with later processing steps that may be required to utilize the fullerenic material. The presence of surfactants can also reduce the effectiveness or functionality of the fullerenic material. For instance, the enhancement of electric conductivity by nanotubes drops sharply when the necessary quantity of surfactant to disperse the nanotubes is present. In addition, sonication may induce defects in the SWCNT and introduce unwanted properties in the SWCNT. Thus, the formation of stable solutions having significant amounts of non-agglomerated nanotubes remains elusive.
The capture of aerosol combustion products, such as amorphous carbonaceous particles, has been performed to help assess their potential health hazardous effect, as well as to study the size distribution of particles at different locations within and above a combustion flame. An aerosol is composed of solid (or liquid) particles in a gas suspension. For purposes of studying particle size, the most important consideration is to avoid altering the particle mass concentration, number concentration, and size distributions by the measuring equipment so that the collected sample at the sampling position has same properties as particles made by an undisturbed flame. Particles made by combustion processes are affected by size-dependent forces such as gravity, diffusion and inertia. For small particles, e.g., less than about 500 nm, diffusion is by far the most important size-dependent force. Diffusion is the net transport of particles from a region of higher concentration to a region of lower concentration caused by the particles' Brownian motion. The relative motion between particles that is caused by diffusion is termed thermal coagulation. Depending on the strength of the intermolecular interactions between particles, thermal coagulation can lead to agglomeration of particles, e.g., clusters of particles. Whether particles will agglomerate depends strongly on collision efficiencies between the particles involved. Because of strong Van der Waals forces between fullerenic materials, particularly nanotubes, thermal coagulation can pose a major challenge for sampling combustion product that is unaffected by agglomeration.